API: rename Index.sym_diff to Index.symmetric_difference

[xflr6]

Following builtin set, I would expect the method name to be symmetric_difference.

How about deprecating sym_diff in favour of this (see also #6016, #8226)?

[TomAugspurger]

@xflr6, well said, We should have included sym_diff -> symmetric_difference along with the diff -> difference change.

Would you be interested in submitting a pull requests to deprecate sym_diff in favor of symmetric_difference?

[xflr6]

Sounds manageable, sure.